This study will determine the efficacy and safety of low dose ridogrel for the maintenance of remission of ulcerative colitis (UC). Patients in this multicenter trial will have achieved remission of their UC while in a previous study of ridogrel in active UC, or will have been in remission for a maximum of 6 months on another medication. They will be randomized to receive oral ridogrel at 0.5, 2.5, or 5.0 mg daily for one year. Patients will be evaluated by subjective symptom evaluation and endoscopic and histologic parameters.